Surface Vibrational Spectroscopy. A Comparison of the EELS Spectra of OrganicAdsorbates at Pt(111) with IR and Raman Spectra of the Unadsorbed Organics

摘要

In this study EELS spectra obtained for the adsorbed species formed from aqueouselectrolytes at Pt(111) electrode surfaces are compared with the IR and Raman spectra of the unadsorbed compounds in order to reveal the changes in vibrational spectra resulting from chemisorption of various important functional groups, and to explore the differences in vibrational absorptivities between EELS spectra of adsorbed species and IR and Raman spectra of the corresponding unadsorbed compounds. Of particular interest are the variations in EELS vibrational frequency, bandwidth and absorptivity due to bonding with the surface, intermolecular interactions of adsorbed molecules and changes in adsorbate molecular orientation (PLD6), benzyl alcohol (BZOH), catechol (CT), benzoic acid (BA), 2-picolinic acid (PA), 2,6-pyridine dicarboxylic acid 926I DCA), and propenoic acid (PPEA). The expected lack of perturbation was found, making TP, BM, and CYS particularly suitable reference compounds for surface vibrational studies of aromatic rings and amino acids. In contrast, L-phenylalanine (PHE) interacts with the PT(111) surface through the phenyl rings as well as the amino acid functionality. In general, chemisorption and intra-layer intermolecular interaction both lower the surface vibrational frequencies (EELS) and incidentally affect peak amplitudes. Orientation affects peak amplitudes and incidentally affects frequencies insofar as a change in orientation is accompanied by a change in mode of surface bonding. Surface roughness leads to a scrambling of adsorbed states which affects bandwidths, chemical/electrochemical reactivities, and in turn the frequencies and amplitudes of EELS peaks. Specific findings and conclusions are presented for each compound.